The study, conducted with researchers from the University of Minnesota and Tsinghua University in Beijing, is published in the most recent edition of the journal Environmental Science and Technology.

The study focused on air quality in China, the world's leader in electric vehicle use. It compared emissions from carbon dioxide and particulate matter from gasoline and diesel cars, diesel buses, electric bikes and electric cars in 34 Chinese cities. Particulate matter means tiny bits of acids, chemicals, dust and metal that can lodge deeply into the lungs.

The research team came up with a formula to measure "per-passenger, per-kilometer" exposure to emissions and particulate matter for each vehicle. To measure emissions for electric vehicles, researchers studied the power plants that produce the electricity.

The results? Diesel cars were the dirtiest, but electric cars were as dirty as diesel buses. Conventional cars were next, with electric bikes being the cleanest. China has an estimated 100 million electric bikes purchased in the last decade.

Cherry cautioned that the results of his study are not necessarily true in the United States, where power plants that generate electricity tend to be much cleaner than those in China.

"Still, all power grids in the U.S. use some form of fossil fuel," Cherry said, "And therefore, electric vehicles are not zero-emissions."

Cherry also found that most power plants in China are outside the cities, while most electric vehicles are inside the cities.

"So you're moving the pollution from the city to the rural area," said Cherry. "Those people that are exposed are not necessarily the users of electric vehicles."